请教PP上一道逻辑！

Ank

Urban air contains more sulfur dioxide than does rural air, and plants in cities typically grow more slowly than do plants in rural areas.  In an experiment to see how much of the difference in growth is due to sulfur dioxide, classes in an urban and a rural school grew plants in greenhouses at their schools and filtered the greenhouse air to eliminate sulfur dioxide.  lants in the urban greenhouse grew more slowly than those in the rural greenhouse.

Which of the following, if true, would it be most important to take into account in evaluating the result?

(A) The urban school was located in a part of the city in which levels of sulfur dioxide in the air were usually far lower than is typical for urban areas.
(B) At both schools, the plants in the greenhouses grew much more quickly than did plants planted outdoors in plots near the greenhouses.
(C) The urban class conducting the experiment was larger than the rural class conducting the experiment.
(D) Heavy vehicular traffic such as is found in cities constantly deposits grime on greenhouse windows, reducing the amount of light that reaches the plants inside.
(E) Because of the higher levels of sulfur dioxide in the air at the urban school, the air filters for the urban school's greenhouse were changed more frequently than were those at the rural school.

为麽选D不可以啊？反正都是解释，说有他因有什么不可以？

bonnie0715

是不是E啊，E说空气要经常换才能保证不被污染，那么这就是一个和对照组的最大不同了。做一个实验，除了需要比较的方面外，其他方面都需要严格相同。（所以我一开始把C也留着，后来比较一下发现大小和空气要空气更严重）。
D陈诉一个事实，无关项。汽车怎样怎样跟这个实验又没关系的。实验已经把所有他因都排除了